The present invention generally relates to a friction material and more particularly, to an improved friction material which may be applied to a clutch lining and a brake lining of an electromagnetic clutch/brake, etc. for use, for example, in an industrial sewing machine and the like.
In the first place, constructions of an electric motor equipped with an electromagnetic clutch/brake, in which a conventional friction material is employed and to which a friction material according to the present invention may be applied, will be described hereinbelow with reference to FIG. 1.
In FIG. 1, the electric motor equipped with the electromagnetic clutch/brake includes an inertia member or fly wheel 3 which is fixedly mounted on one end of a rotary shaft 1 of the driving motor, and to which a clutch plate 2 is secured as one of the constituents of the clutch. In a position confronting the fly wheel 3, there is provided an end bracket 4 which supports a clutch yoke 5 having a clutch winding 5a, a brake yoke 6 having a brake winding 6a and a brake shoe 7, and which has a pulley 9 for deriving output through bearings 8, while a clutch shaft 11 including a sliding shaft 10 (spline shaft) is supported at the other end thereof as shown. On the sliding shaft 10, a clutch ring 14 and a brake ring 15 respectively supporting a clutch lining 12 (friction material) and a brake lining 13 (friction material) are mounted so as to be movable in the axial direction of the sliding shaft 10, but to be prevented from relative movement in the rotating direction thereof, with said clutch ring 14 and brake ring 15 being coupled to each other through an O-ring 16 and a coupling 17.
Subsequently, functions of the electromagnetic clutch/brake having the constructions as described above will be explained hereinbelow.
During the continuous rotation of the rotary shaft 1, rotating energy is accumulated by the fly wheel 3 fixed at the end portion of the rotary shaft 1. Accordingly, upon energization of the clutch winding 5a, electromagnetic force is produced thereby as at 5b, and the clutch ring 14 is attracted to the clutch plate 2, whereby a rotational force is derived from the pulley 9 through the sliding shaft 10 and the clutch shaft 11 via the clutch lining 12 (friction material). For shutting down, upon energization of the brake winding 6a, electromagnetic force is produced as at 6b, and the brake ring 15 is attracted to the brake shoe 7, whereby a braking force is produced with respect to said brake shoe 7 fixed to the end bracket 4 through the brake lining 13 (friction material) via the sliding shaft 10 and the clutch shaft 11 for application of the braking.
The electric motor equipped with the electromagnetic clutch/brake having the constructions as described so far, has been widely employed for driving industrial sewing machines and the like which are generally subjected to starting and stopping very frequently, with the electric motor having a rating of approximately 400 W, and under such conditions for use, the temperature at the surface of the friction material is generally raised up to about 80.degree. C. In the above case, undesirable creaking or squeaking noise tends to be produced during functioning of the friction material in the known friction materials to be employed in the ordinary clutch/brake arrangements. Moreover, since the electric motor equipped with the electromagnetic clutch/brake as described above for driving the industrial sewing machines is normally subjected to the clutching and braking functions in the order of 10,000 times or thereabout per day, the friction material to be employed therefore is required to have a long life sufficient to withstand such repeated operations.
In order to satisfy the requirements as described above, there have conventionally been proposed, for example, in U.S. Pat. Nos. 3,016,119, 3,073,424 and 3,231,058, friction materials for use in electric motors equipped with electromagnetic clutches and brakes, which employ soft porous materials such as compressed cork, baked cork or the like, prepared by mixing particles of cork (obtained by crushing a dried cork tissue of a bark in a soft cell structure) with a particle binding agent such as phenol resin, polyurethane resin, urea resin, polyvinyl acetate resin or the like for subsequent heating and compression. For a mating material corresponding to such friction materials, a carbon steel highly ground or polished is normally employed.
The reason for adopting the soft porous material such as cork for the friction material is such that in spite of the fact that it is a product from a natural plant, for example, cork is chemically stable with respect to fats and oils, organic soaps, alkali, salts, etc. and simultaneously, provides a sufficient resistance against heat as a friction material for the electromagnetic clutch/brake, having a temperature on its frictional surface up to approximately 80.degree. C., without any generation of offensive smell, even by the action of the chemical substances as described above. Furthermore, cork is fundamentally provided with favorable properties such as softness, with a large porosity and a sufficient damping characteristic (tan delta), owing to minute air filled cell structure at a density of approximately 260,000 cells per 1 cm.sup.3, and is suitable as a material capable of reducing or suppressing the undesirable creaking or squeaking noise during application of the clutch and brake, besides its superior friction characteristic.
On the contrary, with respect to friction characteristic of the soft porous material such as cork and the like, the coefficient of friction thereof generally tends to be varied due to external factors such as temperatures, sliding speeds, etc., unless there exists a lubrication film or layer on the frictional surface, thus making the clutching and braking functions unstable. Meanwhile, in the soft porous material such as cork or the like having high coefficients of friction in general, coefficient of static friction is high as compared with coefficient of dynamic friction, and therefore, undesirable sticking and slipping phenomena are apt to take place especially in a region of low sliding speeds, with a large variation in the coefficient of friction due to temperatures, thus resulting in such a disadvantage that the undesirable creaking or squeaking noise tends to be produced during functioning of the clutch and brake. Moreover, since the soft porous material such as cork is inferior in heat dissipation, frictional heat during functioning of the clutch and brake is readily accumulated on its frictional surface, and consequently, abnormal abrasions such as transfer of part of the cork onto the mating material or falling off of part of the cork, etc. may take place. As described so far, in the case where the soft porous material such as cork is subjected to friction directly with respect to the mating material without any intervening lubrication layer, there may arise such problems that the creaking or squeaking noise is generated during application of the clutch and brake or life of the friction material is reduced due to acceleration of abrasion to an extremely large extent.
Accordingly, when a soft porous material such as cork is employed for the friction material, it is essential that the frictional characteristics thereof are improved by forming a lubrication layer on the frictional surface.
For the formation of such lubrication layer, it is effective to impregnate cork with oil or grease as in the friction material disclosed, for example, in U.S. Pat. No. 3,777,864. In the arrangement as described above, it is possible to achieve coefficient of friction particularly stable with respect to external factors such as temperatures, surface pressures, sliding speeds, etc. by the effect of the lubrication layer, and moreover, to obtain the desired coefficient of friction over a wide range by adjusting viscosity of the oily agent forming the lubrication layer. Therefore, as long as the lubrication layer is present on the frictional surface to effect the intended function, transmission of torque during functionings of the clutch and brake is stabilized, with the generation of creaking or squeaking noise being advantageously suppressed.
For the prolonged life of the friction material, which is the outstanding problem in the electric motor equipped with the electromagnetic clutch/brake as described so far, it is necessary that the lubrication layer continuously displays its intended function over a long period.
On the other hand, the friction material as described in the foregoing is generally mounted in the actual motor in the form of an electromagnetic clutch or brake movable plate integrally formed with the clutch ring or brake ring by bonding. More specifically, the movable plates for the electromagnetic clutch and brake which employ the friction material prepared by impregnating a soft porous material such as cork with oil or grease, have conventionally been produced generally by two methods as described below.
In the first method, the friction material prepared by shaping a mixture of cork particles and a binding agent into a desired configuration through heating and compression, is secured to the clutch ring or brake ring by an adhesive, and thereafter, oil or grease is applied onto one frictional surface of the friction material thus fixed.
Meanwhile, in the second method, the friction material prepared by shaping a mixture of cork particles and a binding agent into a required configuration through heating and compression, is, for example, directly impregnated with oil or grease so as to keep at least the cork particles in a state wet with the oil, and the friction material thus treated is fixed onto the clutch ring or brake ring, for example, by an oily adhesive.
However, the conventional practices as described above have such disadvantages that, in the first method, oil or grease can not be uniformly penetrated in a large amount into the friction material, while in the second method, it is difficult to achieve a sufficient adhesion, since the friction material preliminarily impregnated with oil or grease must be bonded to the clutch ring or brake ring, with a consequent low reliability.
Furthermore, for impregnating the soft porous material such as cork or the like with oil or grease as described above, it has been a common practice to apply oil or grease onto the material, or immerse or dip the material in oil or grease, with a simultaneous employment of external physical means such as heating, pressurization, pressure reduction, vibration, etc. However, since cork itself, for example, has a resinous substance, together with the air-filled cell structure as is known in its application to waterproof materials, cork stoppers, etc. it is difficult to cause the cells thereof to be fully impregnated with oil or grease, and such oil or grease is normally penetrated between particles of cork. Therefore, uniform impregnation of the friction material with a large amount of oil or grease is not readily effected in either of the above practices.
The amount of oil impregnated and uniformity thereof as described above, however, are essential factors governing the quality and performance of the friction material, and thus, it has been required to provide a technique which is capable of achieving a long life of the friction material through maintenance of a proper amount of impregnation and uniformity of oil or grease, and simultaneously achieving a reliable adhesion of the friction material with respect to the clutch ring or brake ring.